Merc 4hp 4 stroke

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rbkuconn1
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Merc 4hp 4 stroke

Post by rbkuconn1 » Tue Apr 09, 2019 5:57 am

I have a 12’ Jon boat with a 4 hp Merc 4 stroke for fishing-great little boat.
I have replaced the following:
Carb.-Brand new
Fuel Pump-Brand new
Gas tank-Brand new
Gas lines-Brand new
Plug-Brand new
Fuel filter-Brand new
Fuel squeeze bulb-Brand new
The problem is it runs like a champ for an hour plus, then for no reason, it starts to sputter.
Squeezing the fuel bulb will keep it running. Looking at the fuel filter, it is all but empty when this happens.

Note: This is the second Fuel pump, no change.
Any suggestions would be appreciated.
Rick

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Sideburns
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Re: Merc 4hp 4 stroke

Post by Sideburns » Wed Apr 10, 2019 10:33 am

Welcome to the site! I'll take a stab at it. 3 possibilities come to mind. Sounds like this motor has an external fuel tank? Does this tank have a vent? If its a modern tank, it's likely in the fuel cap. It may be restricted and not letting air into the tank.

The fuel pump will be sucking fuel from the tank, so everything between the tank and fuel pump is under vacuum and any poor connection or air leak could draw air instead of fuel. Check quick connectors, fuel lines under the engine cover etc... Also check fuel pump to crankcase signal hose well.

3rd one is my favorite....Fuel boiling / vapor lock. Fuel pumps are no good at pumping vaporized fuel, fuel lines under the cover could be getting too hot from misrouting, fuel pump mislocated or the engine could simply be overheating. Thermostat, water pump, combustion to cooling water leak etc...

Good luck!
"If it still works, take it apart and find out why!"

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hewesfisher
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Re: Merc 4hp 4 stroke

Post by hewesfisher » Thu Apr 11, 2019 7:31 am

Will echo remarks above though I doubt it's warm enough to cause fuel boiling.

The most likely issue is tank not venting correctly and the very first thing I'd check.

The fuel pump on this engine is a simple diaphragm pump that needs liquid fuel to work, that's why when the problem exhibits and you squeeze the bulb the engine picks back up. A second indicator is the near empty fuel filter. Dollar to donuts you've got a tank venting issue, pinched line, or some other restriction between engine and tank. Don't overlook the fuel line connector either!

Were all parts, in particular, fuel line, bulb, fuel pump, fuel filter, and carb Mercury brand or aftermarket? Matters greatly on some.
Phil

'09 Hewescraft 20' ProV
150hp Merc Optimax
8hp Merc 4-stroke
Raymarine DS600X HD Sounder
Raymarine a78 MultiFunctionDisplay
Raymarine DownVision
Raymarine SideVision
Baystar Hydraulic Steering
Trollmaster Pro II
Traxstech Fishing System
MotorGuide 75# Thrust Wireless Bow Mount

rbkuconn1
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Re: Merc 4hp 4 stroke

Post by rbkuconn1 » Fri Apr 12, 2019 4:54 am

Thanks for the suggestions.
I agree it may be a fuel delivery issue. The new gas tank is self-venting. I did try opening the cap with no change. Also the lines are new-no crimps.
Also, I replaced the connections at the motor and tank-thinking air getting sucked in-no dice.
My next step is to isolate/insulate the fuel lines under the cowling-if this doesn’t work, it may come down to an electrical issue?
Again-thanks

TrackerPro16
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Re: Merc 4hp 4 stroke

Post by TrackerPro16 » Fri Apr 12, 2019 12:01 pm

While Sideburns covered it pretty well, I will second his third ( =D> ) and favorite part about fuel vaporizing. Could be a thermostat that is sticking or, if the engine was run in salt water, the cooling passages getting plugged up. That would all come down to heat. If it gets too hot the fuel system is susceptible to vibration causing bubbling in lines, filters. I would go more with a fuel vapor issue as the filter looks empty.
Electrical could be a problem. If you have a meter that can check inductance you can check the coils, which would be my 'go to', then the trigger coil. Numbers for those can usually be found on 'The Googles'.
What color was the spark plug porcelain tip when you removed it after running? White? Tan? And look down deep into the plug also and tell us if the color is different?
Try changing out the fuel to non ethanol if you don't already run it to see if there is a difference.
Best of luck!

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Sideburns
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Re: Merc 4hp 4 stroke

Post by Sideburns » Fri Apr 12, 2019 2:19 pm

^^^^ yep. Ethanol boils at a much lower temp than gasoline alone. Fuel boiling is probably a less likely scenario than many of the things youve already checked, but because of this, is something that and intelligent person could easily miss. Good call on questioning the ignition system. Ignition coils often fail only when hot.

I should also mention that air in a fuel filter can at times be misleading. A good portion of the filter may have air during normal operation, depending on the angle of the filter. Also, that air bubble will expand normally when the fuel pump pulls a vacuum on the air trapped inside. -But that being said, if there is no air in the filter at the start, and air seems to appear even with a full tank you obviously have an air leak in the fuel line down stream of the filter.

One more note, as hewesfisher was hinting above earlier, just because the part was replaced doesnt mean its good. Some of my biggest blunders in diagnostics have come from bad parts!
"If it still works, take it apart and find out why!"

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hewesfisher
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Re: Merc 4hp 4 stroke

Post by hewesfisher » Sat Apr 13, 2019 9:10 am

Sideburns wrote:
Fri Apr 12, 2019 2:19 pm
One more note, as hewesfisher was hinting above earlier, just because the part was replaced doesnt mean its good. Some of my biggest blunders in diagnostics have come from bad parts!
Yessir.

What year is this Mercury 4hp 4 stroke?
Phil

'09 Hewescraft 20' ProV
150hp Merc Optimax
8hp Merc 4-stroke
Raymarine DS600X HD Sounder
Raymarine a78 MultiFunctionDisplay
Raymarine DownVision
Raymarine SideVision
Baystar Hydraulic Steering
Trollmaster Pro II
Traxstech Fishing System
MotorGuide 75# Thrust Wireless Bow Mount

rbkuconn1
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Re: Merc 4hp 4 stroke

Post by rbkuconn1 » Wed Apr 17, 2019 3:56 am

Sorry-just saw this.
2006

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hewesfisher
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Re: Merc 4hp 4 stroke

Post by hewesfisher » Wed Apr 17, 2019 7:03 am

Your engine is fully compatible for ethanol fuel up to 10%, and JHMO, that is not a contributing factor to your issues. It's my opinion your problem is the new self-venting tank. I'd borrow a different, known good tank and run off it to isolate to the engine or fuel supply.
Phil

'09 Hewescraft 20' ProV
150hp Merc Optimax
8hp Merc 4-stroke
Raymarine DS600X HD Sounder
Raymarine a78 MultiFunctionDisplay
Raymarine DownVision
Raymarine SideVision
Baystar Hydraulic Steering
Trollmaster Pro II
Traxstech Fishing System
MotorGuide 75# Thrust Wireless Bow Mount

rbkuconn1
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Posts: 4
Joined: Mon Apr 08, 2019 6:55 am

Re: Merc 4hp 4 stroke

Post by rbkuconn1 » Thu May 02, 2019 2:24 am

Will try-Thanks

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